1. Technical Field
The present invention relates to optical amplitude modulators for use with fiber-optic cable waveguides. More particularly, the modulator uses an electro-optic material to controllably change the optical phase of two parts of a transmitted and reflected light beam, irrespective of polarization, causing interference and, hence, modulation when the two parts recombine.
2. Background
Great efforts have been made to obtain polarization-insensitive optical devices in fiber-optic communication systems, which include modulators, switches, attenuators, etc. Most of these need a set of polarization pre-process devices, such as TE-TM mode converters, polarization selective couplers, or polarization rotators, to adjust the polarization of the light beams to obtain polarization insensitive devices. For example, one type describes a polarization-insensitive switch which comprises polarization-dependent components advantageously arranged. The polarization insensitive switch is achieved by splitting incoming arbitrarily polarized light waves into two paths, a light wave with a TE radiation component and one with a TM radiation component. The light wave with the TE component is then converted to a light wave with a TM component. Both light waves with TM components are then switched in a polarization-dependent photonic switching device. The initial polarization state is recovered by converting the appropriate switched TM component to one having a TE component. The converted TE component is then combined with the remaining switched TM component in a polarization combiner. These may be difficult to keep aligned.
Another example uses a liquid crystal switch/attenuator using two birefringent polarizers. The first polarizer separates an arbitrarily polarized optical signal into two polarization modes, an ordinary ray and an extraordinary ray. The liquid crystal controls the rotation of the optical signals with respect to the optical axis of the first polarizer with respect to the optical axis of the second polarizer. The second polarizer analyzes the optical signals and sends them to a combiner. The main drawback to this device is that liquid crystal response times may not be fast enough for some telecommunications applications.
Another type of polarization-insensitive optical device is taken from the species of optical reflection modulators with movable mirror surfaces. In this, an end of an input fiber is near the focal point of a lens so that emerging light expands and is collimated to impinge on two mirrors, each covering about half of the expanded beam. By moving the mirrors with respect to each other by a fraction of the light wavelength, the path length in the two halves can be changed so that when the reflected light is imagined onto an output fiber, the two halves of the beam can be made to interfere constructively or destructively. Although the design is elegantly simple, since it uses piezoelectric or electrostatic mirror drivers, it is unlikely to reach the kind of speeds desirable in fiber-optic communication systems. Another problem is that, since the mirrors must be free to move, the structure must be constructed with an air gap so that maintaining alignment with the reflecting surface may be difficult.